In nature, the lithification of unconsolidated materials commonly occurs by the infilling of intergranular void space with interstitial material deposited from solution as mineral overgrowths and cements. This loss of void space progressively decreases the primary permeability and could reduce it to insignificance. A close natural analog of the Natural Analog System (NAS) is the formation of “caliche” soils in the southwestern United States desert regions. Such regions are typically characterized by dry lake beds that are progressively cemented by salts precipitated from the occasional run-off precipitation which reaches the lake basin and then evaporates.
Another example of a natural analog of the NAS is the formation of low permeability “hard pan” soil zones caused by precipitation of cement via ground water evaporation at the sub-surface water table interface. The most common precipitate cement in these examples are calcium carbonate (CaCO3) and various forms of silica (SiO2 or SiOx(H2O)y) as compatible in ambient alkaline or acid environments, respectively. Most ground waters, however, are neutral to alkaline (pH>7.0). NAS is primarily designed for this situation.
The NAS process follows the same principle of reducing void space to reduce permeability by artificially stimulating or inducing void space filling via interstitial precipitation, crystallization, and addition of particulates plus or minus cementation to duplicate that natural process, but much faster.
Related methods of treatment of ground strata include U.S. Pat. No. 4,869,621, issued on Sep. 26, 1989 to McLaren et al. for METHOD OF SEALING PERMEABLE EARTH SURFACE OR SUBSURFACE MATERIALS HAVING ALKALINE CONDITIONS BY INDUCED PRECIPITATION OF CARBONATES. McLaren et al. propose a method of artificially sealing voids in earth strata under alkaline conditions by inducing precipitation, via pumped slurries of aqueous solutions which may include finely divided solids, for example, of calcium carbonate, usually in the form of calcite.
U.S. Pat. No. 4,981,394, issued on Jan. 1, 1991 to McLaren et al. for METHOD OF SEALING PERMEABLE UNCONSOLIDATED MATERIALS. McLaren et al. propose a method for forming solid layers or local zones of material upon or below the earth's surface and above the water table to inhibit the flow of ground waters through such layers of materials.
The NAS process is a method of precipitating calcium carbonate cement in the ground that duplicates natural geologic cementing mechanisms. Calcium carbonate, the artificially produced product of the process, is analogous to the naturally produced calcium carbonate cements of sedimentary rock. A significant potential use, among several, of the NAS process is to reduce or eliminate ground water flow-though in contaminated soils and rocks, and thereby immobilize and isolate such sources of contamination in the natural environment. A principal advantage of the NAS process in environmental remediation and engineering applications is that the cement (calcite) is a natural analog, the permanence of which can be established by comparison with similar naturally calcite-cemented geologic materials. The NAS process introduces the concept of using such natural analog materials in environmental remediation and restoration projects rather than using artificial materials. Such artificial materials can not be assessed in terms of very long-term performance of the projects in various geologic settings.
The principal advantage of the NAS process when used in environmental remediations and restorations is that it can be applied by fluid injection in situ, that is, without excavation and processing of the contaminated-site soil or rock. A contaminated site can be isolated from the ambient ground water and immobilized as a source of hydrologically transported chemical species, without disturbance of surrounding terrain or structures. Further, the subsequent long-term performance of the remediated site can be determined by comparison with naturally occurring carbonate-cemented sites. Where appropriate, the NAS process can be applied by physically mixing NAS process components in contaminated soil and waste material to achieve remediation.
An important aspect of the NAS process is the induced precipitation of ancillary compounds that bind or capture hazardous chemical species from ground water or directly from the waste associated with a contaminated site. Such compounds are analogs of minerals known to be stable (insoluble) in such hydrogeologic conditions. The result is the immobilization of various hazardous chemical species (e.g., lead) into artificial minerals, the subsequent long-term environmental permanence of which can be documented by comparison with the equivalent naturally occurring minerals.
It is an object of this invention to reduce and/or eliminate soil/rock permeability and achieve isolation from ground water flow/pathways in land-fill, hazardous and toxic waste-site, and radioactive waste-site remediation.